Devices, including minimally invasive surgical tools, may be controlled by controlling the motion of multiple rigid bodies forming the device. In machines, mechanisms, robots, etc., multiple rigid bodies are often inter-connected such that one body (body 1) has certain motions or degrees of freedom (DoF) with respect to another body (body 2). These motions or degrees of freedom may be accomplished in one of two ways: via serial design (also known as serial kinematic design, serial kinematic chain, and/or serial kinematic mechanism) or via parallel design (also known as parallel kinematic design, parallel kinematic chain, and/or parallel kinematic mechanism).
As used herein, “Degrees of Freedom” (DoF) is a technical term to convey “motions” in an abstract technical and academic sense. In all, there are six independent degrees of freedom possible between two rigid bodies: three translations and three rotations. A joint will allow anywhere between zero and six DoF between the two bodies. For the case when the joint allows zero DoF, this effectively becomes a “fixed joint” where the two bodies are rigidly fused or connected or attached to each other. From a kinematic sense, the two bodies are one and the same. For the case when the joint allows all six DoF, this effectively means that there is no joint, or that the joint really does not constrain any motions between the two bodies. Any practical joint or mechanism allows 1, or 2, or 3, or 4, or 5 DoF between two rigid bodies. If it allows one DoF, then the remaining 5 possible motions are constrained by the joint. If it allows two DoF, then the remaining 4 possible motions are constrained by the joint, etc.
The technical term “kinematics” may refer to the geometric study and description of motion of bodies relative to other bodies. FIGS. 22A and 24 show the difference between a serial kinematic mechanism and a parallel kinematic mechanism. An abstract representation of a serial kinematic mechanism is shown in FIG. 22A, in which body 1 is connected to body 2 via a serial chain of intermediate bodies. If one traces or scribbles a line from body 1 to body 2, there is only one mechanical path (or line) of motion transmission, which makes this a serial design.
Like body 1 and body 2, the intermediate bodies are also rigid, for all practical purposes (nothing ever is perfectly rigid and sometimes some compliance may be intentional). The connectors are simple or complex joints that may allow certain motions and constrain other motions. For convenience, the teems joint and connector may be used interchangeably. Examples of a connector 2305 would be a simple pivot joint (FIG. 23), also known as a hinge, where the pin 2305 would be the connector between the two bodies 2301, 2303 that are pivoted with respect to each other. A hinge with a pin 2305 connecting the two bodies is one example; the intermediate bodies 2301, 2303 may be connected to the objects that are pivoted relative to each other.
A simple joint such as the one shown in FIG. 23 may allow one rotational DoF and constrains the remaining five. Another example would be a prismatic or sliding joint that allows one translational DoF and constrains the remaining five. Another example would be a ball and socket joint that allows three rotational DoF and constrains the remaining three. Alternatively, the connector could be a flexure joint such as a living hinge. These are only a few examples of connectors that are simple joints. In any of these, there are two bodies and some connector in between.
Any mechanism, for example, the serial kinematic mechanism of FIG. 22A, may be part of a larger device, machine, or even mechanism. As shown in FIG. 22B, the serial kinematic mechanism of FIG. 22A is shown to be between two tabs 2201, 2203. But the first tab 2201 may be fused with body 1 (and therefore first tab and body 1 are one and the same), and the second tab 2203 may be fused with body 2 (and therefore second tab and body 2 are one and the same). Body 1 and body 2 can be part of a larger tool, device, machine, or any other mechanism. In that case, the entire mechanism between tabs 2201 and 2203 may be thought of as a complex joint in the large device, machine, or mechanism.
FIG. 24 shows an abstract representation of a parallel kinematic mechanism. In this example, body 1 is connected to body 2 via multiple independent chains of intermediate bodies. Each such chain represents a mechanical path of motion transmission. If one traces possible lines from body 1 to body 2, there is more than one mechanical path, which makes this a parallel design. The connection paths are not parallel in a geometric sense (e.g. two straight lines being parallel such as the opposing sides of a rectangle), but parallel in the kinematic sense, which implies multiple (more than one), independent, non-overlapping chains or paths between body 1 and body 2. The connectors here are simple or complex joints that may allow certain motions and constrain other motions. For convenience, the term joint and connector may be used interchangeably.
Thus, serial and parallel kinematic mechanisms differ in the number of possible connection paths (intermediate rigid members separated by joints) between two tabs or rigid bodies. Although the individual connectors or joints in serial and parallel kinematic mechanisms are similar, their arrangements (linkage, chains, etc.) are different.
Any mechanism, for example, the parallel kinematic mechanism of FIG. 24, may be part of a larger device, machine, or even mechanism. In that case, the entire mechanism between body 1 and body 2 may be thought of as a connector or complex joint, as shown as “New Connector” in FIG. 25.
Even though a mechanism generally comprises multiple joints, there is a certain equivalence between the terms “mechanism” and “joint”. Both may refer to an apparatus that allows certain motions or DoF between two bodies and constrains the remaining DoFs. While a joint may be used to refer to a simpler construction, a mechanism may refer to a more complex construction (e.g., which may comprise multiple joints).
Refer to FIG. 25, which is an alternative reproduction of FIG. 24. Everything that lies between body 1 and body 2 (all the intermediate bodies and connectors) may be viewed as a black-box and be termed as one “new” connector or complex joint. Thus, what was viewed as a mechanism in FIG. 24 may also be equivalently viewed as a connector or complex joint in FIG. 25. By the same token, any connector shown in the mechanism of FIG. 22A or FIG. 24 may be a simple joint (such as pivot/pin joint or a prismatic/sliding joint) but can also be a more complex joint or a mechanism in itself.
One example of a serial kinematic mechanism is a universal joint, which may include a rigid body, a pin joint, another rigid body, a second pin joint, and a third rigid body. This entire mechanism (comprising all its rigid bodies and joints) is referred to as a “universal” joint. As used herein, a “joint” refers to a mechanical connection that allows motions as opposed to a fixed joint (such as welded, bolted, screwed, or glued joint). In the latter case, the two bodies become fused with each other and are considered one and the same in the kinematic sense (because there is no relative motion allowed). However, when we refer to “joint” in this document, we mean a connection that allows certain motions, e.g. pin (e.g., hinge) joint, a pivot joint, a universal joint, a ball and socket joint, etc. Thus the referenced joint may interface one body with another in a kinematic sense. Yet another academic term for “joint” is “constraint”. Thus, a “connector” or “joint” or “mechanism” or “constraint” allows certain motions or degrees of freedom between two rigid bodies and constrains the rest.
The particular motions that are constrained are also motions that can be transmitted from one rigid body to the other rigid body. That is because since the joint does not allow that particular motion between the two bodies, if one body moves in the constrained direction, it drives along with it the other rigid body as well along that direction. In other words, that particular motion is transmitted from one rigid body to another.
One application area where parallel kinematic mechanisms may be used includes instruments for minimally invasive surgery. Minimally invasive surgical (MIS) and other minimal access procedures are increasing in frequency and becoming more complex, thus demanding improvements in technology to meet the needs of surgeons. In these procedures, generally thin tools are inserted into the body through ports such as trocars or cannulas, which require only small incisions. Motion input from the user, such as a surgeon, is transferred via the tool to the motion of a manipulator or end effector attached to the tool's tip inside the patient's body. This arrangement is used to carry out an operation within the body with the end effector that is controlled from outside the body by a surgeon. This eliminates the need for making large incisions. MIS tools range from simple scissor-like tools to complex robotic systems.
Most traditional tools for use in MIS are mechanical and hand-held, and provide four degrees of freedom (DoF) (three translations and one roll rotation) plus grasping at the end effector, while some newer ones further add up to two DoF (pitch and yaw rotations). These mechanical hand-held tools are inherently capable of force feedback, in general. The traditional mechanical tools are difficult to use because of their lack of dexterity (i.e. the yaw and pitch rotational DoF). While the newer tools are capable of enhanced dexterity given their extra two DoF, they present non-intuitive DoF control (input motion to output motion mapping) schemes that limit user's ability to fully exploit the tool's enhanced dexterity capability. With robotic tools, the user has intuitive control over the dexterity of a tool tip manipulator, the use of electromechanical actuators to produce motion of the tool tip manipulator takes away the mechanical force feedback. In addition, large size, high cost, and limited large-scale maneuverability also reduce the overall functionality of such robotic systems.
Therefore, most existing multiple DoF tools lack the design characteristics to allow for enhanced dexterity as well as desired functionality in a cost effective, compact package. In particular, multiple DoF tools that allow for wrist-like rotations of the tool tip manipulator are important to meet the needs of modern minimal access and MIS procedures, but are not effective unless comfortable, ergonomic, and intuitive control of these additional DoF are ensured.
Examples of serial kinematic mechanisms used in minimally invasive surgical tools may be found in U.S. Pat. No. 5,908,436 to Storz (showing an input joint between a handle and a frame connected by a serial kinematic mechanism) and in U.S. Pat. No. 7,454,268 to Toshiba (also showing a medical device with an input joint between a handle and a frame). In both cases, the input joint is a serial kinematic mechanism. The robotic surgical system shown in U.S. Pat. No. 6,714,839 describes a serial kinematic mechanism as the input joint between a handle and a frame. As used herein a handle is any manual interface (e.g., for fingers, wrist, palm, etc.) and is not limited to controls that are held in the hand. In some of these devices, the frame may refer to a shaft, e.g., tool shaft or an extension of the tool shaft.
In the above cases, the frame is a mechanical reference or a “local ground”. It is not necessarily an absolute ground (i.e. attached or bolted to the actual ground). Rather, the frame serves a mechanical reference or local ground for the handle. In the kinematic sense, one may be interested in the motions or DoF of the handle with respect to the frame, and therefore the frame serves as a mechanical reference. Similarly, handle is to be understood in a generic sense, not simply as something to be “held” in the hand; handle could be something that interfaces with the hand, e.g., the fingers, thumb, etc.
In the examples listed above, the handle has at least two rotational DoF (pitch and yaw rotations) with respect to the frame, provided by the input joint. One challenge of using a serial kinematic mechanism design as the input joint of a surgical tool or machine or device is that of transmitting the two rotational DoF from the input joint to another location on the tool or machine or device. For example, the device of U.S. Pat. No. 5,908,436 to Storz or the device of U.S. Pat. No. 7,454,268 to Toshiba has a serial kinematic mechanism as the input joint that provides the handle with two rotational DoFs (pitch and yaw rotations) with respect to the frame. These two DoF are accomplished via a serial kinematic arrangement of two pivot joints with orthogonal rotational axes. In a practical application the handle may be driven by a hand and the two resulting rotations will be available at two pivot joints. While the axis of one pivot joint (i.e. the first axis) is fixed with respect to the frame, the axis of the second pivot joint (i.e. the second axis) is not. Because of the serial kinematic arrangement, the second axis itself rotates with respect to the frame about the first axis. For the tool, device, or machine to be useful, it is generally desirable or required that the two rotations of the input joint be capture and transmitted (in some cases mechanically) to an end effector (such as a grasper, etc.) at some other location on the tool, device, or machine.
In this case, one can capture the rotation about the first axis relatively easily (e.g., by mounting a pulley at this particular pivot joint), or mounting a gear at this pivot joint location that would rotate with respect to frame about the first axis; the resulting axis of rotation of the gear will remain fixed with respect to the frame serving as its ground. This facilitates a variety of mechanical transmission methods/systems to transmit the rotation about first axis to a remotely located end effector which all operate with respect to the same ground reference frame. Unfortunately, since the second axis itself rotates with respect to the frame about the first axis, it does not remain practical or easy to transmit the second rotation to a remote end effector on the frame. Doing so would require designing and constructing a transmission across a moving interface or pivot joint, the first pivot joint in this case. Designing and building a transmission across any moving interface/joint is non-trivial, and adds significant complexity, cost, and the potential for failure. These are some of the biggest limitations of a multi-DoF serial kinematic mechanism design. One way of overcoming the above challenges is to use an electronic transmission rather than mechanical transmission, similar to how a joy-stick (an input interface to many computer controlled tools/devices/machines) works. Instead of mounting a pulley (or other mechanical means for transmission) at the pivot joints in the serial kinematic mechanism, a potentiometer or optical encoder, or any other rotary motion sensor, may be included at the first and second pivot joints. A rotary motion sensor would transduce the rotational motion into an electrical signal with a known relationship between the two. In this case, the entire body of the rotary sensor mounted at the second pivot joint may also rotate about the first axis, but that is not a problem because the rotation information captured by this sensor in the form of an electric signal can be communicated wirelessly or via wires to a computer or other electronic hardware. Wireless does not require any physical transmission components, and so the drawback of the serial kinematic mechanisms described above are no longer relevant. When using wires for electrically transmitting the electric signals generated by the rotary sensor, one simply needs to manage the wire/cables routing across the moving interface/joint (first pivot joint in this case) which is commonly done. Wires can be miniaturized, folded, insulated, and routed in many creative ways that are practical and cost-effective. As a result serial kinematic mechanisms are common input joints or input interfaces for various computer or electronics based devices, but are somewhat challenging for purely mechanical devices.
One can make a similar argument for when a serial kinematic design is used as an output mechanism or output joint of a tool or machine or device. In this arrangement it is important to determine how to transmit power or motion from the frame i.e. reference ground, where it is available, to the mechanism output i.e. handle and route it through a serial kinematic chain, where components or links move with respect to each other. To do this mechanically is very complicated, challenging, and generally impractical. Instead, one can route the power electrically via cables, or hydraulically/pneumatically via hoses routed to the various motors/actuators at each joint in the serial kinematic mechanisms. As a result serial kinematic mechanisms are common in devices/machines where electrical, electromechanical, hydraulic, or pneumatic actuation is involved, but are challenging as output joints of purely mechanical devices/machines. Even in the former case, one drawback of a serial kinematic design is that the multiple actuators in the device/machine are not all mounted on the frame or the reference ground, and instead most move along with the DoFs. This may make the machine large and bulky and require moving cable connections, which add to cost and machine size. Some examples of a serial kinematic design being used as the output mechanism of a machine include earth movers (which may include hydraulic actuators powered by flexible tubing/hoses that can bend and flex and therefore be routed over moving interfaces/joints).
Described herein are parallel kinematic mechanisms, including in particular parallel kinematic mechanisms used as the input joint in surgical devices, which may address the issues raised above.